1. Field of the Invention
The present invention relates to a sensor, and more particularly to a gas sensor including an oxygen sensor, an NOx sensor and an HC sensor for detecting the concentration of a specific gas component in an exhaust gas discharged from an internal combustion chamber, or a temperature sensor for detecting the temperature of an exhaust gas.
2. Description of the Related Art
Sensors are generally used for controlling the air-fuel ratio of a motor vehicle engine, and include a sensor element having electrical properties which change in accordance with the concentration of a specific gas component in the engine exhaust gas (e.g., JP-A-2007-47093). FIG. 15 shows such a gas sensor (hereinafter, also referred to as a sensor) 1. For example, the gas sensor 1 includes a sensor element (hereinafter also referred to as an element) 21 made of a solid electrolyte having oxygen ion conduction properties, a metal shell (a shell main body) 11 for holding the sensor element 21, and a metallic protective sleeve (an outer sleeve) 81 provided on a rear end (an upper end in the figures) side of the metal shell 11. Terminal metal fittings 51 are disposed within the protective sleeve 81 so as to be electrically connected to respective electrode terminals 25 provided on side surfaces of a rear portion of the sensor element 21. Lead wires 61 are connected to clamping portions (barrels) 57 provided at rear ends of the terminal metal fittings 51, respectively. The lead wires 61 pass through respective holes (holes) 105 provided in a sealing elastic member (a grommet or a packing) 101 at a rear end of the protective sleeve 81 so as to be led out from the protective sleeve 81.
In the sensor 1 shown in FIG. 15, the terminal metal fittings 51 and front end portions of the lead wires 61 which are connected to the terminal metal fittings 51 are disposed within terminal holes (spaces) 75 provided in a terminal surrounding member (also referred to as a separator) 71 which is disposed within the protective sleeve 81. The terminal surrounding member 71 is made of an electrically insulating material such as a ceramic. As shown in a cross-sectional view of FIG. 16, the terminal surrounding member 71 has terminal holes (spaces, and hereinafter, also referred to as holes) 75 which extend through the terminal surrounding member 71 in a front-rear direction while ensuring insulation between the terminal metal fittings 51. The terminal metal fittings 51 are accommodated in the respective holes 75. As used herein, the “rear end” denotes an upper end of the sensor 1 shown in FIG. 15 or components thereof, and the “front end” denotes an end (a lower end) opposite the upper end.
The terminal metal fittings 51 of the sensor 1 shown in FIG. 15 are usually formed by pressing or bending a sheet metal material. The terminal metal fitting 51 includes a terminal connecting portion 53 provided in the hole 75. The terminal connecting portion 53 is a plate spring portion which is formed by bending and folding back a front end portion of the terminal metal fitting 51. The terminal connecting portion 53 is pressed against the electrode terminal 25 provided on a side surface of the sensor element 21 positioned in a hole portion provided at a center of the terminal surrounding member 71 by the spring characteristics of the terminal connecting portion 53. Accordingly, the terminal connection portion 53 is electrically connected to the electrode terminal 25. Although not shown in detail, groove-shaped recess portions 71b are formed at a front end 71a of the terminal surrounding member 71. Further, hooks 59, formed at respective front ends of the terminal metal fittings 51 so as to protrude in a lateral direction, are configured to fit in the recess portions 71b. 
3. Problems to be Solved by the Invention
In the sensor 1 described above, the hooks 59 provided at the front ends of the terminal metal fittings 51 fit in the recess portions 71b provided at the front end 71a of the terminal surrounding member 71. Consequently, even when the sensor 1 is subjected to an external force which pulls the lead wires 61 and the terminal metal fittings 51 in a rear direction or twists them about axes (imaginary axes) with respect to longitudinal directions of the terminal metal fittings 51, the lead wires 61 and the terminal metal fittings 51 exhibit a certain resistance to the external force and the twisting action. However, this structure can not restrict the terminal metal fittings 51 from moving towards a front end side. In addition, the sensor 1 mounted in an operational motor vehicle is subject to various types of vibration. In these circumstances, firstly, in the sensor 1, the terminal metal fittings 51 may move towards the front end side within the terminal surrounding member 71. Consequently, the position of the terminal metal fittings 51 within the sensor 1 is unstable. Namely, when such movement occurs, reliability in electrical connections between the element 21 and the electrode terminals 25 is reduced, which may result in an electrical connection failure (a contact failure).
Secondarily, although the hooks 59 at the front ends of the terminal metal fittings 51 are still held in the recess portions 71b, when the twisting action or rotating action about the imaginary axis acts on the rear ends of the terminal metal fittings 51, a similar problem as that described above may occur. Namely, since the terminal metal fittings 51 are formed long and narrow, when the rotating action acts on the rear ends of the terminal metal fittings 51, it is not possible to effectively avoid the torsion of a portion of the terminal metal fitting 51 closer to the rear ends thereof than the hooks 59 or a twisted deformation thereof. In such case, an electrical connection failure between the element 21 and the electrode terminals 25 may also occur.